You will need Plugin License key, Client ID and Client Secret Key to get started.
-Plugin License Key: You will get Plugin License Key in email after you purchase Formidable Google Spreadsheet Addon.
-Client ID & Client Secret Key : Follow the step by step tutorial mentioned here to get your Client ID and Client Secret Key.
Go to Forms and edit one of your Forms. On the tab click on Settings => Form Actions.
Add the Spreadsheet Action & select spreadsheet & sheet to map sheet headers with a form fields.
NOTE: First row in your sheet are the sheet headers.
That’s it .. Now when you submit the form it will add new contact in Google Spreadsheet.
You can download the plugin from here